Seasonal Dynamics in the Trophic Status of Papyrocranus Afer (Günther, 1868) (Notopteridae) in a Nigerian Rainforest Stream
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Seasonal dynamics in the trophic status of Papyrocranus afer (Günther, 1868) (Notopteridae) in a Nigerian rainforest stream Richard P. KING (1) The stomach contents of Papyrocranus afer comprised primarily midwater invertebrates, allochthonous macro- phyte debris, allochfhonous invertebrates and preyfish. Epiphytic algae, benthic inuertebrates, neustonic invertebrates and miscellaneous insects were secondary components while zooplankton mas of incidental importance. The fish thus utilized food from the three major spafial levels of the stream surface, midwaier and botfom. The diet was dominated by occasional food-types which were consumed during tèss than 3 months of the year. It was closely linked 10 the distributional ecology of the fish in ihe vegetated marginal water biotope where most of the dietaries abound. There was considerable seasonal plasticity in the specific food-types utilized and this resulted in high year-round trophic divers- ity. A wide trophic spectrum and attendant expanded diet breadth were considered as attributes of high foraging success of P. afer. KEYWORDS: Papyrocranus afer - Nigeria - Seasonality - Dietary habit - Fish. RÉSUMÉ L’ÉVOLUTION SAISONNIÈRE DU RÉGIME ALIMENTAIRE DE PAPYROCRANUS AFER (GÜNTHER, 1868) (NOTOPTERIDAE) DANS UNE RIVIERE FORESTIÈRE DU NIGERIA Le contenu stomacal de Papyrocranus afer est constitué d’invertébrés de pleine eau, de débris de plantes el d’insectes allochtones, et de poissons. Les algues épiphytes, les invertébrés benthiques et divers insectes sont des composantes secondaires tandis que le zooplancton n’est qu’occasionnel. Il apparaît ainsi que ce poisson utilise les trois principales zones de son milieu : la surface, la pleine eau ef le fond. Le régime es1 principalemenf opportuniste avec des proies qui sont consomn@es durant moins de trois mois par an. Il est également en relation étroite avec la répartition du poisson dans les zones herbeuses de bordure. On a observé une grande plasticité du régime qui se traduit par une diversité importante sur un cycle annuel. Ce large spectre trophique est considéré comme caractéristique de l’espèce. ?V~OTSCLÉS : Papyrocranus afer - Rivières’- Nigeria - Régime alimentaire - Poissons. (1) Deparlmenf of Zoology, University of Uyo, P.M.B. IOl7, Uyo, Akwa Iborn Siate, Nigeria. Rev. Hydrobiol. trop. 27 (2) : 143-155 (1994). 144 R. P. KING INTRODUCTION southeastern Nigeria (Fig. 1). The stream has a main-channel total length of 53.5 km between its The quality and quantity of available natural food source. in Ikono Local Government Area and where resources influence aspects of t,he life history of fishes it discharges into the Cross River, close to Nwaniba. including inter alia, growth rate, longevity, repro- Ikpa River has a watershed area of 516 km2, of rluc.tive investment,, sexual maturity and fecundity. which 76 km” (14.8 %) of the lower reaches is liable Diet diversity is a measure of t.he degree of complex- t,o annual flooding of the fringing low land riparian ity or specialization in it.ems consumed. Some fish zone during the rainy season. The nonflood zone of species are oligophagous, utilizing a limited number the Upper reac.hes has a basin area of 440 km2 of food resources while others are polyphagous, sub- (85.2 %) and mean dept,h and width of 2.0 m and sisting on wider spect,ra of items. 12.5 m respectively. The adaptat.ive significance of a broad trophic The ent.ire length of the main channel of the spectrum (high diet diversity) is that it ensures a stream lies at the interface of two different geolog- const,ant# energy source, facilitates adequate utiliza- ical deposits: tertiary sediment.ary rocks and cretace- tion of available food resources and enables the fish ous deposits (see geological map of Nigeria in CLAU- to easily swit.ch from one food source to another in SEN, 1964). The stream is considerably shaded by response to nat,ural pulses in their relative abund- overhanging canopy of riparian vegetation (mostly ances and availability (KING, 1993). Conversely, Elaeis guineensis, Raphia hookeri, Raphia vinifera oligophagous fishes (low diet. diversit.y) would be ad- and other tropical forest trees). The aquatic macro- versely affected should there be a dwindle or collapse phytes are mainly Nymphaea, Vossia and Crinium in t.he food resource-base. species. The seasonal variation in stream surface Tropical riverine fishes inhabit dynamic aquatic temperature is 23.3-28.0 “C, Secchi disc transpar- environments and are thus exposed to seasonal ency 12.5- 101 cm and pH 6.6- 7.7. Conductivity pulses in hydrometeorological attributes. Seasonal- varies between 13.0 and 69.2 PS cm-l. The stream ity in habitat conditions influences fishes mainly current velocity is 3.5 - 6.9 cm s-1. through qualitative and quantitative changes in The climate of the area is typical of tropical rain- available food resourc.es (LO\VE-MCCONNEL~, 1987). fore&; it comprises dry (November - February) and Seasonal variations in dietary status and diversity of wet (March - October) seasons. Meteorological data fishes presumably stem from the inherent dynamic.s from the University of Uyo meteorological station in food resources availability / abundance and/or sited within Ikpa River basin (Fig. 2) indicate the intrinsic changes in foraging ethology e.g. active pre- precipitation and ambient temperature regimes dur- dilection for specific food-types. In any aquatic eco- ing this study. The dry season was characterized by system, t.he evolutionary adaptations should there- prevalence of dry tropical continental winds from fore favour fishes that have evolved trophic the Sahara desert and low mean monthly precipita- strategies that ensure optimum foraging under a sea- tion; peak dry season occurred in January - Febru- sonally available food resource condition. ary. The wet season was typifled by prevalence of Thr present study was aimed at documenting the moist tropical maritime winds from the Atlantic overall food composition and extent of temporal Ocean and high mean monthly rainfall. Annual rain- plasticity (monthly and seasonal patterns) in dietary fa11 was 255.8 cm. The cyclic hydrological regime is status and diversity of Papyrocranus afer in a Niger- typified by high water level and slow rate during the ian rainforest stream. It cent,eri on the influence of rains and vice-versa in the dry season. the tropical dry-wet. season cycle on the seasonality More descriptive information on the Ikpa River regimes of these trophic attributes. Very litt.le has basin is provided by KING (1989) and TEUGELS et al. heen published on the general biology and ecology (1992). of P. afer. Brief and fragmentary accounts of aspects of its reproductive and food habits are contained in ALB~RET (1982) and TEUGELS et al. (1992) respectively. MATERIALS AND METHODS Monthly samples of Papyrocranus afer were obtained from each of eight stations in Ikpa River STU DY AREA (Fig. 1) for 5 days during mid-month (November, 1988-October, 1989 inclusive). Fishing was con- The present study was conducted in Ikpa River, a ducted by the use of 3 gill-nets with 30 mm, 50 mm small perennial rainforest tributary stream located and 80 mm stretched mesh respectively and 50 sin- West of t.he lower reaches of the Cross River in gle-hook (size No. 2) set-lines baited with earth f&w. Hydrobiol. trop. 27 (2) : lz3-15.5(1994). SEASONAL TROPHIC STATUS OP P. AFER (NIGERIA) 145 . \ I !ï015’N I 0 Area liable to flood - - - - Limit of Ikpa River basin -.. l Sampling stations Esuk Inwang Towns Streams 0 5 IOkm l 1 I FIG. 1. - Map of Ikap River, showing the sampling stations. Inset : map of Nigeria showing the locat,ion of Ikpa River. Carte de situafion de la rioière Ikpa et des sfations de pêche, avec les zones inondables (hachurèes). worms. Attemps to capture P. afer with the valued the potentially available food resources to stream basket traps were unsuccessful. Day and night fish- fishes. This is similar to WHYTE'S (1975) taxonomie/ ing were conducted in the open water, littoral zone ecological groupings of the diets of the ilshes of Lake and fringing swamps. Bosumtwi, Ghana. The merits of this system of cate- The specimens were transported to the laboratory gorization of food in flsh dietary studies are dis- where they were measured to the nearest 0.1 cm cussed by BERG (1979). total length (TL). They were eviscerated and the The frequency occurrence of each item as dom- stomachs slit open. The stomach contents of eac.h inant (by volume) (di) and non-dominant (fi) stomach specimen were placed in a Petri dish and aggregates contents were noted. The integrated importance of dispersed with a few drops of water prior to macro- each item was then expressed by the food ponderal scopie and microscopic (with variable magniflcations index (FPI) (KING, 1991): up to x 40) examinations. The contents were sorted, FpI = (ft + 4) identified and categorized according to taxonomie 7 x 100 (1) and microhabitat (i.e. where an item occurred most 2 (fi+&) commonly) criteria based on author’s field notes on i-1 R~U. Hydrobiol. trop. 27 (2) : M-155 (1994). R. P. KING in n compared months/seasons. The IBD ranges from 0 (for completely different set of dietaries in each month/season) to 100 o/. (for identical sets of dietar- ies in each month/season). An index of diet diversity (F) was computed from the o/O FPI data using the formula (see ALATALO, 1981; GRUNDEL, 1990): -1 F = ( i$lW1- [=PC-!iZa PJ - 1 (4 i-1 1 where Pi = proportion of the diet comprised by re- source type i and n = number of food c.at.egories in t.he diet. This index is sensitive to changes in two attributes viz food richness and equitability (= the 60 - degree to which a11items are equally represented); it 50 - increases as food richness and it,s equit.ability increase and declines when few items dominate the 40 - E diet.